Abstract

Like organisms, cohesive social groups such as insect colonies grow from a few individuals to large and complex integrated systems. Growth is driven by the interplay between intrinsic growth rates and environmental factors, particularly nutritional input. Ecologically inspired population growth models assume that this relationship remains constant until maturity, but more recent models suggest that it should be less stable at small colony sizes. To test this empirically, we monitored worker population growth and fungal development in the desert leafcutter ant, Acromyrmex versicolor, over the first 6 months of colony development. As a multitrophic, symbiotic system, leafcutter colonies must balance efforts to manage both fungus production and the growth of the ants consuming it. Both ants and fungus populations grew exponentially, but the shape of this relationship transitioned at a size threshold of 89 ± 9 workers. Above this size, colony mortality plummeted and colonies shifted from hypometric to hypermetric growth, with a distinct stabilization of the relationship between the worker population and fungus. Our findings suggest that developing colonies undergo key changes in organizational structure and stability as they grow, with a resulting positive transition in efficiency and robustness.

Keywords

Development AcromyrmexDivision of labor Colony growth Leafcutter ant Social allometry

Communicated by W. T. Wcislo

Electronic supplementary material

The online version of this article (doi:10.1007/s00265-013-1632-4) contains supplementary material, which is available to authorized users.

Notes

Acknowledgments

Many thanks to John Weser, Bob Johnson, and Diana Wheeler for the help with queen collections, to Kimberly Shaffer for the help with colony care, to Tate Holbrook and Leah Drake for the idea development, and to Jon Harrison, Ulrich Mueller, and two anonymous reviewers for the feedback on the manuscript. Funding for this project came from National Science Foundation DDIG grant 1011171 awarded to RMC and JHF.